Power Quality: Harmonic Issues
John Ging
Transmission Access Planning
What are Power System Harmonics?
Harmonics result in a power system waveform distortion
– Signals occurring at multiples of the fundamental 50Hz frequency are
superimposed onto the waveform
– Destroys the smooth sinusoidal curve
3 Phase
50HzSupply
signal
3 Phase
50Hz
with
andsignal
Harmonic
first
Supply
4 multiples
Distortion
1
50Hz
100Hz
150Hz
200Hz
1
0.8
0.8
R
0.6
0.6
0.4
0.4
0.2
0.2
S
0
-0.2
-0.2
-0.4
-0.4
-0.6
-0.6
T
-0.8
-0.8
-1
-1
0
0.002
0.004 pi/2 0.006
0.008
5pi/6
0.01
pi
7pi/6
0.012
Tim eradians
(s e conds )
0.014 3pi/2 0.016
0.018
0.02
2pi
What are the harmonic sources?
• Caused by the non-linearity of customer loads.
– A result of devices drawing non-sinusoidal current
from a sinusoidal voltage source
– These non-linear loads generate harmonic currents
which interact with the system impedance instigating
a harmonic voltage distortion.
• Commonly generated by rectifying devices
Power Electronics
Power converters
Variable frequency drives
DC motor controllers
Cycloconverters
Static Var Compensators
Steel mills
Power supplies, UPS and Battery Chargers
Inverters
ARC devices
Fluorescent lighting
ARC furnaces
Welding machines
Why are Harmonic problems
becoming an increasing concern?
• Increasing use of power electronics and converters
(particularly for wind turbines) are injecting more
harmonic currents onto the system
• The influx of more cables onto the network exacerbate
the problem by changing the network impedance
• Cables amplify any harmonic injections present
EirGrid have been analysing power system harmonics
for a number of years
• Not a problem that is unique to Ireland
 EirGrid have discussed the issues and mitigation with other TSOs
What does this mean?
Harmonic related distortions can result in the
failure or mal-operation of end-use equipment
–
–
–
–
–
–
Increases the current in the system (i.e. higher losses)
Overheating in transformers, motors and neutral wires
Improper operation of Protection relays
Can resonate with system Capacitor Banks
Telecommunications interference
Misfiring from incorrect triggering at “zero-crossings”
• i.e. multiple crossings of the x-axis can wreak havoc on the
timing signals used for Digital Circuits; Rectifiers, Thyristors
and Inverters
Harmonic disturbances
50Hz
50Hz
signal
signal
with
and
Harmonic
triggering
Disturbance
on x = 0 axis
and
crossings
triggering
1
0
-1
0
0.01
0.01
0.004 0.020.006
0.03
0.008
0.01
0.04
0.0120.05 0.014 0.06
0.016
Tim e (s e conds )
0.018
0.07
0.02
0.08
How can harmonics be mitigated?
This is a complex issue with several options available:
– Analysis to determine the most cost-effective and enduring solution
– Interactions with neighbouring projects increase complexity
– One or more options from the table below may be required
Option
Details
Issues
System Reinforcement
Increase the Short Circuit Level in
the Region
Potentially expensive,
Possibly long lead times,
Requires knowledge of permanent
connection of offers
Isolation transformers
Some higher frequencies can be
attenuated by certain transformer
winding configurations
Not applicable for all harmonic orders,
Potentially expensive
Harmonic injection
limitation
Reduce injections from plant by
enforcing limits
Some plant perform better than others for
minimising harmonic injections, can
potentially be the cheapest viable solution
Passive Filters
Tuned inductive reactors and a
capacitance attenuate select
frequencies
Depends on system impedance which
changes over time,
Requires knowledge of permanent
connection of offers
Active Filters
Automatically tuned reactors and
capacitance attenuate select
frequencies
Expensive,
Stable operation against impedance
changes but may interact with nearby
projects
Grid Code Requirements
CC.10.13 Power Quality
– Users shall ensure that their connection to the Transmission
System does not result in the level of distortion or fluctuation
of the supply Voltage on the Transmission System, at the
Connection Point, exceeding that allocated to them following
consultation with the TSO.
– Distortion and fluctuation limits are outlined in IEC/TR3
61000-3-6 (Harmonics) and IEC/TR3 61000-3-7 (Voltage
fluctuation). Users shall also operate their Plant in a manner
which will not cause the requirements contained in CENELEC
Standard EN 50160 to be breached.
What are EirGrid doing?
• Background harmonic data being recorded at various
stations for model building and verification
• Studies for select nodes with significant cable build are
underway (South West, North West)
• Further studies will be compiled after offer acceptance
(and cable request modifications are received)
• Harmonic limits will be defined with suitable margins
retained as appropriate for future system integrity reasons
• Harmonic Voltage Limits and Impedance loci will be
ascertained and communicated with relevant customers
– Theses limits can be used to select turbines with suitable injections
– The Impedance loci are useful for filter design to ensure the limits are satisfied
What must customers do?
Prior to receipt of Harmonic Injection Limits and Impedance loci
Contact the System Operator well in advance of any potential cable request
modifications
Notify the System Operator of turbine types or reactive power devices on site
as early as possible
After obtaining Harmonic Injection Limits and Impedance loci from the
System Operator, some or all of the following may be required
Purchase/Install turbines that have limited harmonic injections at the specified
frequencies
Install harmonic mitigation plant such as filters to reduce harmonic injections
below acceptable levels at the specified frequencies
Communicate the parameters of the solution devices with the System Operator
More details can be found in our
Harmonics Information note
www.eirgrid.com/aboutus/publications